The standard multi-phase induction motor is one branch of the dynamoelectric machine (DEM) family which may be said to have reached a stage of mature development. Because of the nature of the induction motor and its underlying principle of operation, the basic designs of this type of dynamoelectric machinery tend to be relatively standard.
Usually the construction of an induction motor consists of a stator and rotor mounted within a frame work in such a manner that the rotor is free to rotate in a set of bearings mounted therein.
The stator is connected to a multiphase source of ac power to produce a rotating magnetic field which passes through the rotor. The rotor when subjected to the rotating magnetic field has a structure which produces an induced magnetic rotor field which results in a torque being produced in the rotor which causes rotation of the rotor.
The rotor of induction type dynamoelectric machines whether a standard ac induction motor or an induction start synchronous type ac machine utilizes a construction which follows a standard pattern. A magnetic core or other type of magnetic body formed of laminations of a magnetic material is securely mounted on a shaft or spider in such a manner as to ensure rotation therewith.
In the case of an induction motor, the rotor construction usually consists of a shaft on which packets of disc shaped laminations are mounted in such a manner as to produce a rotor core. Periodically ventilation ducts are produced in the rotor core by the insertion of suitable spacer devices between the lamination packets.
The laminations forming the rotor core are usually provided with axially extending slots at or near the periphery of the rotor for the insertion of rotor bars. In the construction of a fabricated rotor, rotor bars are driven into the rotor slots or recesses so that a portion of each rotor bar extends slightly beyond the ends of the rotor core. In the usual construction of an ac induction type rotor, a shorting ring (usually copper or an alloy thereof is brazed to the rotor bar ends protruding from each end of the rotor core. This construction is usually referred to as a "squirrel cage" rotor.
A similar construction is employed in amortisseur windings of synchronous ac dynamoelectric machines. Designers of this type of machine have for many years employed a "damper" or amortisseur winding to provide the necessary torque to start these machines (most synchronous machines embody a construction which produces zero torque at rotor standstill).
As a result a rotor construction is employed which basically utilizes a squirrel cage type operation within the synchronous rotor to produce the necessary starting torque for the synchronous machine. Rotor bars are driven through recesses provided near or at the surface of the pole faces of the rotor so that the ends of the rotor bars protrude slightly beyond the ends of the rotor poles. A pair of shorting rings are suitably connected (by brazing) to the ends of the rotor bars to permit the flow of current in the rotor bars to produce the resulting rotor magnetic field necessary to produce the rotor starting torque. Because of the maturity of the design of this type of machine, the rotor bars are usually aluminum or copper or alloys thereof, and in the case of an induction motor the rotor usually takes the form of a cylinder.
Although the rotors of synchronous machines take other shapes depending on whether the poles are salient or not, the principles of squirrel cage operation are equally applicable to this type of machine and although this application will generally describe ac induction type motors, the techniques employed to overcome the problems of prior art induction type dynamoelectric motors are equally applicable to dynamoelectric machines of the synchronous type wherein an amortisseur winding is employed to produce starting torque.
The operation of a squirrel cage is fairly well understood and at stand still the operation of the squirrel cage motor may be loosely compared to a transformer. The stator in which the squirrel cage rotor rotates produces a rotating magnetic field which produces an induced electromotive force (EMF), in the stationary rotor which causes circulating currents to flow in the rotor bars of the rotor. At stand still, the frequency of the induced EMF in the rotor bars is equal to the frequency of the electrical supply to the stator windings of the motor. It is in this stage (i.e. locked rotor) that very large circulating currents pass through the rotor bars and shorting rings. The circulating currents will decrease as the rotor speed increases but the currents continue to be large in magnitude as long as the rotor is producing maximum torque. During this period rotor bar currents flow in such a manner that maximum current density occurs at the top of the rotor bars. As the rotor speed increases, the frequency of the induced rotor currents decreases and the current distribution in the rotor bars becomes more uniform.
Under certain conditions which are not completely understood, sparking occurs in squirrel cage rotors of induction motors embodying this type of rotor. Considerable study has been directed to the determination of the cause of the generation of sparks in the rotor of squirrel cage induction machines and one school of thought postulates that although rotor bars are thought to be contacted by each lamination forming the slot surrounding the rotor bar, such contact between rotor bar and successive laminations may not be perfect. As a result, small gaps between the rotor bar and surrounding laminations may exist where several successive rotor laminations may not make a good electrical contact with that particular portion of a rotor bar. It is believed that at these gaps, a sufficient EMF may be built up to cause a spark to occur across the gap even though the rotor bars effectively function as a short circuit. Another theory is the rotor bars have a limited amount of freedom to move and vibrate in the clearance provided in the slot causing a "make and break" contact between the punchings and the rotor bars as the bars move. It is during these conditions when the frequency of the induced EMF in the rotor is highest (i.e. locked rotor) that sparking seems to occur in the rotor.
It is to overcome the occurrence of sparking in squirrel cage rotors of induction motors that this application is primarily directed. However, the technique to be described is equally applicable to the amortisseur windings of synchronous DEM's to prevent amortisseur sparking during operation.